The mutagenicity of chemicals in certain strains of salmonella typhimurium has been shown by B.N. Ames to correlate with the carcinogenicity of these chemicals in animals: 90 percent of the mutagens were carcinogens and 90 percent of the carcinogens were mutagens. Since there has been shown to be a high incidence of renal pelvic tumors in abusers of the analgesic phenacetin, we examined the potential mutagenesis of N-hydroxyphenacetin and its sulfate ester. The sulfate ester of N-hydroxy-2-acetylaminofluorene, a polycyclic analog, has been postulated to be the ultimate carcinogen of 2-acetylaminofluorene. N-hydroxyphenacetin and its sulfate ester were not mutagenic. The sulfate ester of N-hydroxy-2-acetylaminofluorene was less mutagenic than the parent compound incubated with the soluble fraction alone. This indicates that the sulfate ester of N-hydroxy-2-acetylaminofluorene may not be as important in the carcinogenesis of this compound as previously believed or that the Ames' test is less responsive to highly chemically reactive metabolites than it is to relatively stable metabolites.